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Science: Why promiscuity sometimes pays off

8 February 1992

The purpose of every sperm is to find an egg. Biologists know that sperm
follow chemical trails, a phenomenon known as chemotaxis. Now it seems they
‘sniff’ their way to their target, detecting critical chemicals with their
olfactory-like receptors.

Marc Parmentier of the Free University of Brussels and his colleagues
in France and Holland were carrying out a study of human sperm when they
stumbled across a family of receptor molecules, whose structures are uncannily
like olfactory receptors discovered recently in rat nasal tissue. The receptors
are produced by sperm and become embedded in their outer membrane (see last
week’s Nature, p 453).

Parmentier and his colleagues found more than 20 genes coding for these
receptors, which are variations on a molecular theme. The fact that sperm
have so many of these genes implies that sperm may follow many chemical
signals as they journey to the egg, say the researchers.

The discovery that the sense of smell and effective fertilisation have
a lot in common may have practical consequences. For instance, defects in
the genes may adversely affect fertility. And it may be possible to develop
a new method of contraception by altering genes, say the researchers.

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In another piece of sperm research, Australian and Swedish biologists
studied the copulation behaviour of adders in the south of Sweden. Female
adders are promiscuous, mating with as many as eight males. Males may gain
reproductive advantage by being promiscuous; they may have many offspring.
But promiscuous females do not increase the number of their offspring.

Thomas Madsen of the University of Lund, Sweden, and his colleagues
say that by mating frequently female adders may instead be increasing the
quality of their offspring. When a female adder mates she may store the
sperm in her body for several months. By mating with several males, she
sets up competition between the different sperm. The sperm best equipped
to find and penetrate the egg will win. And if there is a correlation between
a sperm’s ability to reach the eggs and the overall quality of the genes
it carries, then the female’s offspring have genetic advantages.

Madsen and his colleagues believe there is such a correlation. Their
studies show that the offspring of females mate several times are physically
healthier than those from single mating (last week’s Nature, p 440).